# Reverse lookup (/en/docs/rust/reverse) solar_dates_by_bazi and reverse_chart - the functions and types for recovering candidate birth dates from BaZi pillars or chart features. Recover candidate birth dates from four BaZi pillars or from chart features. Both entry points are "pruned enumeration + full re-charting", so results have zero divergence from forward charting. Concepts, how pillars follow the Config boundaries, and the multi-solution / truncation semantics are on the [reverse lookup guide](/en/docs/guide/guides/reverse). ```rust use x_iztro::*; let cands = solar_dates_by_bazi( (HeavenlyStem::Geng, EarthlyBranch::Chen), (HeavenlyStem::Jia, EarthlyBranch::Shen), (HeavenlyStem::Bing, EarthlyBranch::Wu), (HeavenlyStem::Geng, EarthlyBranch::Yin), (1900, 2100), &Config::default(), )?; ``` Everything is defined in `x_iztro::astro::reverse` and re-exported at the crate root. ## Types [#types] ### BirthCandidate [#birthcandidate] One candidate birth moment, ready to hand to [`by_solar`](/en/docs/rust/astro). | Field | Type | Meaning | | ------------ | -------- | ------------------------------------------------------ | | `solar_date` | `String` | solar date, `YYYY-M-D` | | `time_index` | `u8` | hour index 0–12 (0 = early Zi hour, 12 = late Zi hour) | ### StarPosition [#starposition] A star and the branch of the palace it sits in: the atomic condition of a feature lookup. | Field | Type | Meaning | | -------- | --------------- | -------------------------------------------------------------------------- | | `star` | `StarKey` | the star (natal chart stars only; horoscope-scope flow stars are rejected) | | `branch` | `EarthlyBranch` | the branch of its palace | ### ReverseCriteria [#reversecriteria] The condition set of a feature lookup. Implements `Default`; the idiomatic construction gives the conditions and closes with `..Default::default()`. Every condition is optional, but at least one must be given. | Field | Type | Default | Meaning | | --------------------- | --------------------------- | -------------- | -------------------------------------------------------------- | | `soul_branch` | `Option` | `None` | soul palace branch | | `body_branch` | `Option` | `None` | body palace branch | | `five_elements_class` | `Option` | `None` | five elements class | | `stars` | `Vec` | empty | star placements, all of which must hold | | `mutagens` | `[Option; 4]` | all `None` | which star carries each birth-year mutagen \[Lu, Quan, Ke, Ji] | | `year_range` | `(i64, i64)` | `(1900, 2100)` | inclusive solar year range, within 1583–9999 | | `fix_leap` | `bool` | `true` | leap month correction, same meaning as the charting parameter | | `limit` | `usize` | `0` | candidate cap; `0` takes `DEFAULT_REVERSE_LIMIT` | ### ReverseResult [#reverseresult] | Field | Type | Meaning | | ------------ | --------------------- | ------------------------------------------------------------------------------------------ | | `candidates` | `Vec` | the birth candidates satisfying every condition | | `truncated` | `bool` | whether the search stopped early at the candidate cap; later solutions were never searched | ### DEFAULT\_REVERSE\_LIMIT [#default_reverse_limit] ```rust pub const DEFAULT_REVERSE_LIMIT: usize = 512; ``` The candidate cap used when `ReverseCriteria::limit` is 0. *** ## solar\_dates\_by\_bazi [#solar_dates_by_bazi] Recover solar birth dates from four BaZi pillars. ```rust pub fn solar_dates_by_bazi( yearly: (HeavenlyStem, EarthlyBranch), monthly: (HeavenlyStem, EarthlyBranch), daily: (HeavenlyStem, EarthlyBranch), hourly: (HeavenlyStem, EarthlyBranch), year_range: (i64, i64), config: &Config, ) -> Result, IztroError> ``` The pillars are interpreted under the boundary readings of `config` (`year_divide` for the year pillar, `horoscope_divide` for the month pillar, `day_divide` for the late Zi hour) — the same semantics as the `raw_dates.chinese_date` a charted astrolabe reports, so reversing any chart's pillars always includes that chart's birth moment. A set of pillars recurs roughly every 60 years within the range; an hour branch of Zi may yield two candidates on adjacent days because of the early/late Zi hour split. **Example** ```rust let a = by_solar("2000-8-16", 2, Gender::Female, true, Language::EnUS, Config::default())?; let p = a.raw_dates.chinese_date; let cands = solar_dates_by_bazi(p.yearly, p.monthly, p.daily, p.hourly, (1900, 2100), &Config::default())?; for c in &cands { println!("{} {}", c.solar_date, c.time_index); } ``` **Output** ```text 1940-8-31 2 2000-8-16 2 2060-8-1 2 ``` **Errors** A pillar with mismatched stem/branch polarity (such as 甲丑 Jia-Chou — a yang stem on a yin branch), or a year range that is reversed or outside 1583–9999, returns [`IztroError::InvalidArgument`](/en/docs/rust/errors#invalidargument). *** ## reverse\_chart [#reverse_chart] Recover candidate birth dates from chart features. ```rust pub fn reverse_chart( criteria: &ReverseCriteria, config: &Config, ) -> Result ``` Judgement runs entirely under `config`: the mutagen table, the school and every boundary follow it, so charting a candidate with the same `config` is guaranteed to satisfy every condition. Chart layout does not depend on gender (gender only affects the direction the decadal horoscope advances), so the criteria carry no gender. **Example** ```rust let r = reverse_chart( &ReverseCriteria { soul_branch: Some(EarthlyBranch::Wu), five_elements_class: Some(FiveElementsClass::Wood3rd), stars: vec![StarPosition { star: StarKey::ZiweiMaj, branch: EarthlyBranch::Wu }], mutagens: [Some(StarKey::TaiyangMaj), None, None, None], year_range: (1998, 2002), ..Default::default() }, &Config::default(), )?; println!("{} {}", r.candidates.len(), r.truncated); ``` **Output** ```text 39 false ``` **Errors** Empty criteria, a horoscope-scope flow star in `stars`, or an invalid year range returns [`IztroError::InvalidArgument`](/en/docs/rust/errors#invalidargument). Reaching `limit` stops the search; later solutions never appear in the result. On `truncated = true`, narrow `year_range` or add conditions and query again.